This invention pertains to methods and apparatus for locating and tracking vehicles and, more particularly, to methods and apparatus for locating and tracking vehicles using a cellular telephone network.
Vehicle locating and navigating systems fall into three categories. The first category, known as dead-reckoning systems, encompasses systems which employ an electronic monitor mounted on the vehicle which constantly tracks the movement of the vehicle's wheels to determine direction and distance. This information is transmitted to a control station which relates the information to the vehicle's initial position, as indicated in the memory of a computer at the control station.
The second category encompasses systems which employ homing techniques. Examples of such systems are disclosed in U.S. Pat. Nos. 4,818,998 and 4,908,629 to Apsell et al. In these systems, a transmitter is mounted on the vehicle which emits a constant signal upon activation. A tracking vehicle, such as a police cruiser, "homes in" on the signal source using a directional antenna to determine the direction of the signal's greatest strength.
The third category encompasses systems which use triangulation or trilateralization (also known as "arculation") to determine the vehicle's location. These systems employ several land or airborne base stations (such as satellites) whose position is known and which transmit signals to, or receive signals from, the vehicle. A parameter of the signal is used to determine either the distance between each of the base stations and the vehicle, or the direction from each of the base stations to the vehicle. These data then are used for triangulation or trilateralization to determine the vehicle's position.
U.S. Pat. No. 4,891,650 to Sheffer discloses a vehicle locating and tracking system which uses the base stations of a cellular telephone network. The system comprises a mobile transmitter located on the vehicle which transmits an alarm signal upon activation to detectors located at the base stations of the network. These detectors receive the transmitted signal and transmit, to a central station, a corresponding signal indicating the strength of the received signal and the identity of the base stations receiving the signal. A computer at the central station uses these data to determine the distance between the vehicle and each of the base stations and, through trilateralization, to determine the vehicle's position.
U.S. Pat. No. 3,680,121 to Anderson, et al. also discloses a vehicle locating and tracking system using the base stations of a cellular telephone network. A transmitter located on the vehicle transmits a signal which is modulated onto a carrier for transmission to the base stations. Detectors located at the base stations determine the phase of the modulating signal with respect to a synchronization signal transmitted from a local television station. The differences in phase of the received modulating signal from at least three base stations are used to calculate the distances between the vehicle and these base stations. These data then are used to determine the vehicle's position.
An article by D. Sweeney, Stolen Vehicle Recovery (Cellular Business, September 1990) discloses a vehicle locating system of Code-Alarm Company which employs a device located on the vehicle for receiving signals from the base stations of the LORAN navigational network. This network transmits pulsed signals over large sections of the globe, primarily for use by ships in navigation. By measuring the differences in the times of arrival of these pulses from several base stations, the position of the vehicle is determined. The device then transmits the position-location information to a central station over a cellular telephone network.
In practice, all of these systems have presented problems. Dead-reckoning systems are expensive, accumulate errors over time and require frequent recalibration.
Homing systems require the active participation of the police or private security personnel for the operation of a tracking vehicle. These systems, moreover, generally are effective only over line-of-sight distances.
Although systems employing methods of triangulation or trilateralization can be more accurate and do not require the intervention of a tracking vehicle, these systems are prohibitively expensive if a dedicated system of base stations is required.
Although the systems disclosed in U.S. Pat. Nos. 4,891,650 and 3,680,121 employ an existing network of cellular telephone base stations, these systems nevertheless require substantial additional equipment. Dedicated circuitry must be installed at each base station to receive and interpret signals from the vehicle's transmitter and to transmit corresponding signals in response to these signals. Also, particularly in the case of stolen vehicle recovery, it is difficult to obtain sufficient power from a small, hidden transmitter for interaction with a sufficient number of cellular base stations to accurately determine location. For example, a typical transmitter of a cellular telephone normally has the capacity for communication with only the nearest base station within the network. These systems, moreover, present problems of accuracy because of multipath interference of the signal transmitted from the vehicle.
The system of Code-Alarm Company, disclosed in the article by D. Sweeney, Stolen Vehicle Recovery, also employs an existing network of base stations, the LORAN navigational network. The Code-Alarm system attempts to avoid the problems of transmitting a signal from the vehicle, for the determination of distance, by using signals transmitted from the base stations for this determination. The parameter of the signals used is the differences in the times of arrival of these signals at the local receiver in the vehicle. The LORAN network, however, is not well suited for precise location of a vehicle in, e.g., a large city. This network is designed for long-range navigation and, without a prohibitively large receiving antenna on the vehicle, provides a relatively coarse resolution.